Late-stage peptide nitroarylation: stereoelectronic tuning and molecular recognition

Aromatic amino acids lie at the center of protein folding and interactions. For the three aromatic amino acids commonly found in proteins - phenylalanine (Phe), tyrosine (Tyr), and tryptophan (Trp) - their large size and hydrophobic character, as well as electrostatic interactions involving their π systems, provide substantial energetic driving forces for folding and interactions. Beyond Phe, Tyr, and Trp, an increasing number of non-canonical aromatic amino acid residues have been explored in both rational and screening approaches for new peptide-based drugs and materials. These efforts are critically dependent on relatively expensive amino acid building blocks, highlighting a need for new approaches to rapidly generate peptide libraries featuring non-canonical aromatic amino acids starting from less expensive building blocks. Here we describe a new approach for site-specific introduction of stereoelectronically diverse aromatic amino acid side-chains into peptides. The key step - nitroarylation of short, nucleophilic side-chains with o-fluoronitroarenes - efficiently produced dozens of nitroarylated peptides from just two peptide intermediates. We demonstrate broad compatibility with standard solid-phase peptide synthesis reagents, including in the context of clinically relevant peptides. Nitroaryl side-chains display significant near UV and/or visible absorption for specific detection, and peptides bearing these side-chains were efficiently recognized and hydrolyzed as protease substrates. Chemoselective nitro reduction on fully deprotected peptides further expands the accessible chemical space to electron-rich aryl side-chains. In providing parallel access to dozens of stereoelectronically diverse aromatic amino acids from shared peptide intermediates, this approach permits rapid peptide library synthesis for structure-function studies, including peptide drug and materials discovery.